Update automatic preview documentation

[k-finken]

Documentation changes

Update automatic preview documentation to reflect that previews are only created for PRs targeting deploy branch. Stems from this conversation: https://mintlify.slack.com/archives/C0922GQ5Z09/p1764008105386429?thread_ts=1763142495.246939&cid=C0922GQ5Z09

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• [ ] Code examples work as written
• [ ] Commands and configurations are correct
• [ ] Links resolve to the right destinations
• [ ] Prerequisites and requirements are accurate

✅ Clarity and completeness

• [ ] Instructions are clear and easy to follow
• [ ] Steps are in logical order
• [ ] Nothing important is missing
• [ ] Examples help illustrate the concepts

✅ User experience

• [ ] A new user could follow these docs successfully
• [ ] Common gotchas or edge cases are addressed
• [ ] Error messages or troubleshooting guidance is helpful

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[!NOTE] Clarifies that automatic preview deployments are only created for pull requests targeting the deployment branch.

• Docs:
  • Update deploy/preview-deployments.mdx to state automatic previews are only created for pull requests targeting the deployment branch.

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